The characteristics of hot-wire type intake air quantity sensors change depending on substances deposited on the surface of the sensors. Deposition of the substances causes an error in the quantity of fuel to be supplied to an engine, whereby there arise problems of deterioration of the exhaust gas and reduction in operational performance. To cope with the problems, the hot wire has been heretofore heated at a temperature higher than a temperature in the normal operation of the engine after the engine has been stopped to thereby burn off the deposit. A method of burning-off of the deposit is disclosed, for instance, in Japanese Unexamined Patent Publication No. 76182/1979.
It is revealed from experiments that temperature for heating the hot wire should be about 1000.degree. C. in order to obtain an effective burning-off operation. However, when the hot wire is heated at 1000.degree. C., a fuel-air mixture is possibly fired. The firing of the fuel-air mixture is inconvenient for the operation of the intake air quantity sensor disposed in an intake passage for a gasoline engine. Heretofore, in order to avoid the firing of the fuel-air mixture, the burning-off operations have been carried out only when the temperature of the intake passage and the revolution of the engine satisfy predetermined conditions during its operation and the fuel-air mixture excessively supplied to the intake passage during warming-up operations for the engine is sufficiently scavenged. Alternatively, the burning-off operations have been carried out before the fuel-air mixture reversely flowing from a fuel supplying portion after the engine has been stopped reaches the intake air quantity sensor, the reversing time of the mixture being obtained by experiments.
However, various experiments revealed that the burning-off under the above-mentioned conditions have not provided a sufficient result because the firing of the fuel-air mixture has been often caused. Namely, under the condition of continuous burning-off operation due to uncontrolability of a burning-off control circuit, an output from the intake air sensor in a period from starting of the engine to an idling operation becomes several tens times as large as an output in the normal operation. In such condition, an amount of the fuel to be supplied to the engine is about several tens times as normally required. Accordingly, there is no complete combustion of the fuel, and a large amount of the fuel remains in the intake passage. This may cause the firing of the fuel by the hot wire heated at about 1000.degree. C.
There is another problem in the detection of the output from the intake air sensor. Namely, when the engine is restarted during the burning-off operation or immediately after the completion of the burning-off operation, the output of the sensor is reduced to be nearly zero volt in a period (about 5 seconds) before the temperature of the hot wire returns to the normal operation temperature.
If the output of the intake air quantity sensor is detected at that moment, to examine whether or not the sensor is faulty, erroneous judgment will be given. Further, when the fuel is to be supplied under the condition of an abnormal output of the sensor, an erroneous air-fuel ratio will be given.